When coronary arteries, narrowed by disease, cannot supply the heart muscle with sufficient blood, the cardiac circulation may be improved by grafting a section of vein from the leg to bypass the obstruction. Around 10,000 people in the United Kingdom have this operation annually and the results are usually good. It is a major procedure that lasts several hours and requires the heart to be stopped temporarily, with blood circulation and oxygenation taken over by a heart-lung machine.